Electrical machines are used in vehicles, for example, as a motor or starter for an internal combustion engine or, for example, as a generator for generating electricity. In modern vehicles, electrical machines are also used as a combination of motor and generator, as a so-called “starter generator” (SG) or boost recuperation machine (BRM). These are electrical machines that can be operated in a vehicle as an electric motor or as a generator, as necessary.
As a generator, starter generators must be able to handle all the tasks conventionally assigned to the generator, namely supplying electricity to the vehicle electrical system and charging the vehicle battery. As an electric motor, when starting the internal combustion engine starter generators must quickly bring the engine's crankshaft to the rotation speed necessary for starting.
Electrical machines constituting boost recuperation machines can be used in particular in hybrid vehicles in order to be operated in both motor mode and generator mode. At low rotation speeds at which an internal combustion engine is not yet providing its full torque, it can be assisted by operating the electrical machine in motor mode. Upon deceleration of the vehicle, operating the electrical machine in generator mode can convert kinetic energy into electrical energy which can then be stored.
Electrical machines can be cooled with an air cooling system. A particular challenge is presented by high power densities together with relatively high ambient temperatures in the engine compartment. These temperatures can exceed 120° C., at which the thermal tolerance limit of individual components can be exceeded.
In order to avoid damage from excessive temperature, temperature sensors can be installed in a housing of the electrical machine. Alternatively, the temperature of a power converter associated with the electrical machine can be sensed and monitored using temperature sensors. Temperature sensors can furthermore be installed directly in windings of the electrical machine.
A winding temperature T can be ascertained from a resistance measurement R(T)=U/I and a knowledge of the temperature dependence of the resistance. This requires an accurate measurement of the current through the winding, however, which is complex and for which no measuring arrangement is usually present.
A measurement of the particularly critical rotor temperature is thus not possible. An attempt is therefore made to deduce the rotor temperature, by way of a model, from the result of measuring the stator temperature. This procedure is susceptible to error. The thermally highly stressed rotor of the electrical machine is very difficult to access for direct temperature measurement. Known signal transfer methods, for example inductive transfer units or slip ring-assisted systems, are usually eliminated due to high cost and insufficient operating reliability.
A need therefore exists for simplified temperature measurement in particular for the rotor of an electrical machine.